The present invention relates to high energy density dielectric ceramics based on lead magnesium niobate (also known as PMN). More particularly, the dielectric ceramics comprise lead magnesium niobate with barium titanate (also known as BT) and/or strontium titanate (also known as ST) additions. Dopants such as tantalum, lanthanum, tungsten and the like may also be included. The dielectric ceramics possess extremely high energy densities and are useful in many applications, including medical devices such as defibrillators and pacemakers. The present invention also relates to a new method of preparing dielectric ceramics, including the use of hot isostatic pressing (HIPing) in an oxygen-containing atmosphere.
Dielectric materials used in ceramic capacitors must have characteristics such as high dielectric constant, low temperature coefficients in the dielectric constant and small dielectric loss. In addition, high energy density capacitors are required to have high permittivities that are relatively stable with temperature and field, high breakdown strengths and minimal field-induced strains.
A known ceramic having a high dielectric constant is barium titanate with additions of stannates, zirconates and other titanates. Strontium titanate and lead lanthanum zirconium titanate are also known as dielectric ceramics. Other conventional ceramics having high dielectric constants comprise solid solutions of lead-containing constituents such as lead magnesium niobate, lead zinc niobate and lead titanate. Examples of such compositions are given in U.S. Pat. Nos. 4,339,544 to Sakabe et al and 4,977,485 to Mori et al.
The methods of preparing conventional dielectric ceramics typically include the mixing of oxide starting materials, followed by sintering. In the manufacture of multilayer capacitors, a layer of metal paste is placed on a green sheet of the ceramic material. Several sheets are then stacked and fired to form the multilayer device. Since the dielectric ceramic and electrode metal are fired simultaneously, various attempts have been made to lower the sintering temperature of the ceramic in order to allow the use of relatively inexpensive electrode metals, such as Ag, instead of expensive, higher melting metals, such as Pt and Pd.
U.S. Pat. No. 4,818,736 to Yamashita et al discloses a high dielectric constant ceramic composition comprising lead zinc niobate, lead magnesium niobate and lead titanate in which a portion of the Pb site of the lead zinc niobate is substituted with Ba or Sr.
U.S. Pat. No. 5,059,566 to Kanai et al discloses a high dielectric constant composite material. One component of the composite may comprise lead zinc niobate, lead magnesium niobate and lead titanate, while the other component is a glass such as B.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3, BaO or MgO.
U.S. Pat. No. 4,724,511 to Alexander et al discloses a high dielectric constant ceramic composition comprising lead magnesium niobate, lead zinc niobate, lead zirconate, titanium dioxide and bismuth titanate, along with other oxide or rare earth additions.
U.S. Pat. No. 4,536,821 to Wheeler et al discloses a high dielectric constant ceramic composition comprising lead magnesium niobate, lead zinc niobate and an oxide such as silica, manganese dioxide, zinc oxide, nickel oxide, alumina, ceric oxide, lanthanum oxide, tungsten oxide, gallium oxide, titanium dioxide or lead oxide.
U.S. Pat. No. 4,265,668 to Fujiwara et al discloses a high dielectric constant ceramic composition comprising lead magnesium niobate, lead titanate and, optionally, other oxides such as lead manganese tungstate or lead manganese niobate. A portion of the lead may be replaced with Ba, Sr or Ca.
U.S. Pat. No. 4,542,107 to Kato et al discloses a dielectric ceramic composition comprising lead magnesium niobate, lead zinc niobate and lead iron tungstate, with minor additions of manganese oxide.
U.S. Pat. Nos. 4,751,209 to Yokotani et al and 5,006,956 to Kawakita et al disclose high dielectric constant ceramic compositions comprising lead magnesium niobate, lead nickel tungstate and lead titanate, with additions of either metals such as Ca, Ba and Sr or oxides such as PbO and NiO.
The dielectric ceramic compositions cited above are not considered useful for high energy density capacitor use. Conventional high energy density capacitors typically have energy densities of about 1 J/cm.sup.3 and include liquid impregnated polymer film capacitors for high voltage applications and electrolytic capacitors for low voltage applications.
The present invention has been developed in view of the foregoing and to overcome the deficiencies of the prior art.